(a) Field of the Invention
The present invention relates to a process for producing a block copolyamide having effectively improved impact strength by anionic polymerization.
(b) Description of the Prior Art
It is well known that lactams can be anionically polymerized in the presence of an anionic polymerization catalyst and certain activators. Many kinds of molded polyamide articles are obtained by anionic polymerization procedures. In general, these anionically polymerized polyamide articles possess high mechanical strength, good abrasion resistance and good fatigue resistance due to high degrees of crystallinity. However, the impact strength of these articles is fairly low and this disadvantage causes the application of anionic polymerization to be restricted.
In order to improve impact strength, it is proposed to employ processes for producing a block copolyamide from lactams and polyols such as polyalkylene glycols, polylactonediols and polyolefin polyols by anionic polymerization.
One of the important advantages of anionic polymerization is that final products can be directly obtained by polymerizing raw materials such as lactam and polyol in a mold. Therefore a high polymerization rate is demanded for the effective production of molded products. In general the polymerization rate in the anionic polymerization of lactam is mainly governed by the structure of the employed activators. There have been presented no activators which enable completion of polymerization of a mixture of lactam and polyol in a short time and also produce molded articles having good surface appearance and mechanical properties. It haas been long desired to create activators which satisfy all of the above-described requirements.
For example, when isocyanate compounds were employed as activators (U.S. Pat. No. 3,704,280), it took a very long time to complete the polymerization reaction because of low activity of the activators. As another example, when polyacyllactams such as adipyl biscaprolactam, terephthaloyl-biscaprolactam or isophthaloyl-bis-caprolactam were used (U.S. Pat. No. 3,862,262), molded products could not be demolded easily and, therefore, had poor surface appearance. Surface appearance can be improved to some extent by increasing the amount of activators, but then, decrease in molecular weight and impact strength of the produced polymer occur simultaneously.